Test a photocell- HSI type heater
to the manufacturer, DESA International:
Connect the leads of an ohmmeter to the terminals on the photocell assembly. Use the 0 to 20,000 OHM scale on the meter. The resistance should measure 11,000 OHMS or less. If not the photocell is defective and should be replaced.
Block off the light source by putting thumb over opening of photocell. Replace the photocell if a change in resistance is not observed.
The resistance should measure greater than 50,000 OHMS (use the 1 megohmscale on the meter).